This invention relates generally to the field of digital broadcast communication through the medium of free space, and more particularly, to the field of digital audio broadcasting in spectral regions relying on line-of-sight communications subject to signal fade due to multipath propagation.
In recent years, the quality of commercial audio broadcast signals as delivered by radio transmitters through free space has been eclipsed by the quality of stored program material such as compact digital audio disk technology and digital audio tape technology. The quality differential of stored program material over conventional analog frequency modulated broadcasting is so great that there has been a market shift in listener preference to stored program material. In the past, commercial frequency modulated broadcast signals were considered comparable in quality to stored program material. However, the increase in quality of stored program material and the degradation in fm signal quality, particularly in urban areas, has resulted in a differential in quality that is reflected in decrease in listenership and substantial revenue loss to broadcasters.
One of the reasons for degradation in quality of fm signals is the inherent problems with line of sight propagation of radio signals. Radio signals are subject to interference and fading from specularly-reflected copies of the signal, both narrow-band and broadband. The interference is commonly referred to as multipath. Multipath results in fade which is dependent upon frequency, time and spatial factors. However, a free space signal is generally unaffected by group delay, which is generally associated with other types of media such as copper wires, coaxial cables, fiber optics and waveguides. A careful analysis of the propagation problem in connection with other limiting factors, such as other available spectrum and compatibility with other broadcast services, has revealed that no system exists or is even been suggested as suitable to address the problem of delivering a high-quality broadcast signal of sufficient efficiency and practicality to be commercially viable.
One digital audio broadcasting scheme promoted as potentially viable is designated the Eureka 147 Project. Based on an implementation in the 1500 Hz microwave L-band, the Eureka 147 system proposes a single-transmitter, thirty-three channel system having a 7 MHz bandwidth and relying on an error-correcting convolutional encoding and decoding scheme. Implementation of such a system would involve substantial economic dislocation within the U.S. broadcast market and further, would require that the power be as much as 500 times higher than current analog broadcasting system power to operate comparably at the frequencies of interest. Of technical interest, the Eureka 147 system as proposed would not be able to share spectrum with any other broadcast service, and the use of the system would require that there be a central transmitter in each market and thus that each market be limited to thirty-three channels. The scheme, as proposed, is not particularly spectrum efficient, requiring guard bands between channels to mitigate against intersymbol interference. The system is intended to be multipath resistant. To this end there is a broad spectrum allocation of 7 MHz and a digitally-based error correction scheme.
The problem of reliable transmission and reception in the presence of severe multipath propagation has been explored in the past. For example, Cimini, Jr., reports in a paper entitled "Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing," IEEE Transactions on Communications. Vol. COM-33, No. 7, pp. 665-675 (July 1985), that broadband, narrow-band and time-dependent multipath problems in a harsh mobile environment are a substantial challenge to solve. The problems addressed therein presuppose the application of speech/data transmission on a frequency carrier at 850 MHz. Simulations were performed based on orthogonal frequency division multiplexing (a plurality of subcarriers or multitone modulation) with the 7.5 kHz channel bandwidth. Detection was based on coherent detection using one or more pilot signals transmitted in the same manner and at the same amplitude as the information signals. It was assumed that the co-channel interference would be at least 17 dB below the information carrier signals. Cimini identified that reliable retrieval of pilot signals is essential for accurate coherent detection and correction of data. Proposed specifically was a dual pilot system with interpolation in frequency between the pilots and orthogonal frequency division multiplexing wherein each tone is modulated by a quadrature phase shift keyed (QPSK) modulator. It was therein recognized that quadrature phase shift keying had a 3 dB advantage in gain over differential quadrature phase shift keyed modulation and detection.
Error correction techniques are well known for correcting errors in digitally-modulated information signals. The use of error correction techniques provide substantial advantage over conventional analog signaling because detection of less than a complete portion of the signal can still result in recovery of all of the information intended to be communicated. The exact form of error correction and encoding plays a significant role in the efficiency and integrity of the transmitted signal. It is important for example that the selected error correction technique be one which does not so degrade the channel efficiency as to render it impractical.
Multiple-channel digital transmission systems have been known in various environments. For example, the U.S. Military RAKE System is described in "Modern Communication Principles," by S. Stein and J. J. Jones (McGraw Hill, 1967). Other references on multi-channel digital transmission systems include: D. L. Nielson, "Microwave Propagation and Noise Measurements for Mobile Digital Radio Application," SRI Report for ARPA SRI Project 2325, NTIS No. ADB073934C (January 1975); W. W. Peterson, E. J. Weldon, Jr., Error-Correction Codes, MIT Press (1973); and Shu Lin, An Introduction to Error-Correcting Codes, Prentice Hall (1970).
Multiple-channel digital modulation techniques are also known and have been used in telephone line modem technology, such described in U.S. Pat. No. 4,206,320 to Keasler et al. (June 1980), U.S. Pat. No. 4,438,511 to Baran (March 1984), U.S. Pat. No. 4,601,045 to Lubarsky (July 1986) and U.S. Pat. No. 4,754,458 to Dornstetter (June 1988) . The last-named patent is believed to describe a technology in the proposed Eureka 147 system. The Dornstetter patent describes the use of a digital modulation scheme capable of correcting certain types of errors and specifically, burst errors. The system uses convolutional coding and a decoding algorithm based on a Berlekamp-Massey algorithm. The error correction scheme is based on the calculation of coefficients of code words by obtaining polynomials from the factorization over a Galois field of elements. The result is a relatively inefficient use of bandwidth.
An instructive survey article on multicarrier modulation is by John A. C. Bingham, entitled "Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come," IEEE Communications Magazine, Vol. 28, No. 5, pp. 5-14 (May 1990). This article deals with many of the problems related to multicarrier modulation and identifies areas requiring further development. The principal focus of use of multicarrier modulation has, however, been in connection with communication through impaired channels, such as telephone lines. Frequently, in such environments, there is two-way communication to promote retransmission of lost information.
What is needed is a one-way (broadcast) transmission system which is efficient and sufficiently robust to be able to communicate high-fidelity audio program material in a manner which is compatible with existing U.S. broadcast services, particularly fm broadcast services.